Interpretation of the veiling of the photospheric spectrum for T Tauri stars in terms of an accretion model

Abstract

The problem on heating the atmospheres of T Tauri stars by radiation from an accretion shock has been solved. The structure and radiation spectrum of the emerging so-called hot spot have been calculated in the LTE approximation. The emission not only in continuum but also in lines has been taken into account for the first time when calculating the spot spectrum. Comparison with observations has shown that the strongest of these lines manifest themselves as narrow components of helium and metal emission lines, while the weaker ones decrease significantly the depth of photospheric absorption lines, although until now, this effect has been thought to be due to the emission continuum alone. The veiling by lines changes the depth of different photospheric lines to a very different degree even within a narrow spectral range. Therefore, the nonmonotonic wavelength dependence of the degree of veiling r found for some CTTS does not suggest a nontrivial spectral energy distribution of the veiling continuum. In general, it makes sense to specify the degree of veiling r only by providing the set of photospheric lines from which this quantity was determined. We show that taking into account the contribution of lines to the veiling of the photospheric spectrum can cause the existing estimates of the accretion rate onto T Tauri stars to decrease by several times, with this being also true for stars with a comparatively weakly veiled spectrum. Neglecting the contribution of lines to the veiling can also lead to appreciable errors in determining the effective temperature, interstellar extinction, radial velocity, and v sin i.

On the nature of the spectrum veiling for classical T Tauri stars in the near infrared

“…According to Muzerolle et al (2003) emission of dust component of protoplanetary disk gives noticeable excess relative to the photospheric radiation only at wavelengths λ > 2 µm, so to explain observed veiling near 1 µm Fischer et al (2011) have suggested that there is an additional source of the continuum emission with temperature 2500-5000 K. Gahm et al (2008) found from the analysis of highly veiled CTTS's spectra that emission lines, which are formed in the accretion spot along with continuum, contribute significantly to decreasing of photospheric line's depth. Dodin & Lamzin (2012) confirmed this discovery by theoretical modeling and also demonstrated that "veiling by lines" is important for both CTTS with high and small veiled spectra. Moreover they found that at given effective temperature of the star T ef the relative contribution of lines to veiling increases with decreasing of accretion flux, which is defined by the relation F ac = ρ 0 V 3 0 /2, where ρ 0 and V 0 are pre-shock gas density and velocity respsectively.…”

“…As the result dependence of the total (lines+continuum) veiling on wavelength in optical band becomes non-monotonic instead of monotonic in the case of veiling by continuum only -see Fig. 7 in Dodin & Lamzin (2012).…”

“…Intensity of hot spot's continuum emission in Dodin & Lamzin (2012) model decreases when one moves from visible to NIR spectral band as well as in the models of Calvet & Gullbring (1998). However wavelenght dependence of veiling by lines should be different, because it depends on gas temperature and density distribution in the stellar atmosphere heated by accretion shock radiation, as well as on parameters of individual lines, rather than on hot spot's effective temperature.…”

“…We will consider in this paper spectra of the system "star+round homogeneous spot", calculated by methods, described by Dodin & Lamzin(2012). The only difference is that we consider here not only visible but also NIR spectral band, more precisely from 0.45 to 1.2 µm, i.e.…”

“…We will see below (see also Dodin & Lamzin, 2012) that r λ strongly varies from one line to another, and therefore lines, used for veiling measurements, should be always specified. Unfortunately until now nobody does it, interpreting the scatter of r λ -values of individual lines in the considered spectral band as a "random error" of measurements.…”